A packaged focal plane array (FPA) may comprise a detector array, an associated reference detector array, a read-out circuit, and a transmissive cover that seals one or more of these structures. The cover may be sealed via a seal structure, such as a seal ring. The seal structure may comprise two components, a first component positioned on the transmissive cover, and a second component positioned on the substrate containing the detector array, reference array, and read-out circuit. The two components may be aligned and soldered together to seal the two components together.
In certain instances, one of the surfaces of the transmissive cover may include an infrared shield that may be made of the same metal (e.g., gold) used to form the two components of the seal structure. Accordingly, the solder used to seal the infrared detector wets to the shield just as it does the seal structure. If the solder migrates out from the seal structure and wets to the shield, the shield may draw more solder onto the shield. This may deplete the solder from the seal and prevent a hermetic seal. Solder extrusion may also cause short-circuiting in the reference detector array. In addition, if molten solder extrudes from the seal area and contacts the silicon surface, a reaction may occur between the silicon and the tin in the solder. The reaction products may degrade the bond line and the reliability of the hermetic seal.
One common method of preventing the solder from wetting to the shield is to make the dimensions of each shield only slightly larger than the dimensions of the corresponding reference detector array. The seal structure is then dimensioned so that there is a sufficient gap between the seal and the shield to prevent solder extruded during a bonding process from wetting to the shield. Extending or providing a barrier layer beyond the area of the seal may also prevent contact between the silicon and the solder.